The fine particles of a metal, in particular, fine particles of gold, platinum, palladium, rhodium or silver have a possibility to be used as a medicine utilizing penetrating ability of an ultra fine particles into reticuloendothelial system, an inspection agent with function, material for Drug Delivery System (DDS), an improving material for detection sensitivity at the analysis utilizing the surface sensitization effect by resonance Raman scattering, and methods for preparation of dispersion of said metals which does not coagulate and is stable are presently being investigated.
In the above mentioned circumstance, as the substantial methods for the preparation of nano size particles e.g. gold up to the present time, following 1-4 methods are proposed.
1. The method to reduce metallic ion by coexisting surfactant or polymer.
This method is characterized by coexisting surfactant or polymer in the reducing reaction medium aiming to prevent the flocculation of metal generated by the reduction of the metallic ions and to stabilize the fine particles of metal generated by the reduction of metallic ions. This method is characterized by coexisting surfactant or polymer with metallic ions and using reducing agent such as sodium borohydride (NaBH4) separately from the reduction by light irradiation from the outside. According to this method, although the reduction of various metals can be possible, the problem that it is necessary to add a reducing agent for each metal ions separately is pointed out. [Document 1; Schmid, Chem. Rev., 92, 1709 (1992)].
2. Method for stabilizing using gold/sulfur reciprocal action This method is the method for stabilizing of fine particles of gold generated by reduction utilizing strong affinity between sulfur and gold, by bonding fine particles of gold electrostatically with sulfur atom. In this case, the sulfur atom which displays above mentioned function can be provided by introducing a group having said molecule at the end or side chain of a unit of arbor shape molecule (dendron) or polymer. It is known that a DNA molecule is also effective for the stabilization of fine particles of gold instead of above stabilizing substance. The merit of this method is that the surface of metal particle can be modified by various organic compound, especially by a compound having desired functional group, however, it is not effective to the metals except gold. [Document 2; M. Brust, M. Walker, D. Betheli, D. J. Schiffrin, and R. Whyman, Chem. Commum, 1994, 801. Document 3; V. Chechik and R. M. Crooks, Langmuir 15, 6364 (1994). Document 4; M.-K.
Kim, Y.-M. Joen, H.-J. Kim, S. G. Hong, C. G. Park, and K. Kim, Chem.
Commum, 2001, 667.)].
3. Method for preparation of fine particles of metal using dendrimer as a template
This method is the method for preparation of fine particles of metal of several ten nm size using water soluble poly(amide amine) dendrimer, characterized by using a reducing agent such as sodium borohydride (NaBH4) after metal ions are introduced into the dendrimer. [Document 5; M. Zhao, L. Sun and R. M. Crooks, J. Am. Chem. Soc, 120, 4877 (1998). Document 6; L. Balogand D. A. Tomalia, J. Am. Chem. Soc., 120, 7355 (1998). Document 7; Y. Nie, L. K. Yeung and M. Crooks. J. Chem. Soc., 123, 6840-6846. Document 8; L. K. Yeung and M. Crooks. Nano Lwtt., 1, 14-17 (2001.)].
The merit of this method is that the fine particles of metal obtained by reduction of metallic ions can be of monodispersed particles with narrow distribution based on the function of the template, however, it is necessary to add a reducing agent.
On the contrary,
4. The method to obtain the stabilized dispersion of fine particles of metal by reducing metallic ions utilizing the reducing ability of poly(dithiafulvene) and dispersion stabilizing ability of fine particles of metal is known. This method is to reduce metallic (golden) ions to fine particles of metal by stirring effectively polythiafulvene and metallic (golden) ions in dimethylsulfoxide. By this method it is not necessary to add reducing agent of metal from the outside, and utilizing the reducing ability which the polymer originally has, and this is the characteristic of this method (Document 9; Y Zhou, H. Itoh, T. Uemura, K. Naka and Y. Chujo, Chem. Commum., 2001, 613-614.). However, this method is necessary to carry out the reaction in dimethylsulfoxide, and since the generated fine particles of metal are stabilized by poly(dithiafulvene), the generated fine particles of metal are soluble in dimethylsulfoxide. And, the necessary use of dimethylsulfoxide as the medium, causes the problem to the environment.
The subject of the present invention is to provide a method for preparation of metallic nano particles which dissolve the problems which above mentioned prior arts have. Said problems can be illustrated as follows. Namely, the problem that the reducing agent for metal must be added beside the component which stabilize the metallic nano particles, and the problem that, when an inorganic reducing agent is not added separately, since it is necessary to use organic medium to obtain stabilized metallic nano particles, the obtained metallic nano particles can not be used for the reaction in water. In other words, the subject of the present invention is to provide a method for preparation of the metallic nano particles having the characteristic of monodispersion, which is possible to reduce metallic ions without using an inorganic reducing agent and using water as the medium, by generating metallic nano particles, which are stable in water, under coexisting of the material which has a characteristic of template.
The inventors of the present invention already proposed that the shell cross-linked micelle (SCM) can be synthesized by following process, that is, synthesizing block copolymer of polysilane, which is hydrophobic polymer, and polymethacrylic acid, which is hydrophilic polymer containing carbon, forming micelles in water based on the amphipathicity, then by cross-linking a carboxylic group of shell part of the polymer micelle by diamine compound. [Document 10; T. Sanji, Y Nakatsuka, F. Kitayama and H. Sukurai, Chem. Comun., 1999, 2201-2202. Document 11; T. Sanji, Y. Nakatsuka, S. Ohnishi and H. Sakurai. Macromolecules, 33. 8524-8526 (2000 Further, it is also well known that the shell cross-linked micelle is completely water soluble.
In the meanwhile, it is publicly known that polysilane has a reducing ability for metallic ions [Document 12; A. F. Diaz, M. Baier, G. M. Wallraff, R. D. Miller, J. Nelson, W. Piero, J. Electrochem. Soc. 138,742 (1991)], further, it is also publicly known that the metallic layer can be formed by utilizing said polysilane layer as a reducing ability of metallic ions such as Au, Ag, Pt or Pd for forming metallic layer [Document 13; M. Fukushima, N. Noguchi, M. Aramata, Y Hamada, E. Tabei, S. Mori and Y. Yamamoto. Syth. Met., 273-280 (1998)].
However, there is no technical paper reporting the method to obtain the dispersion of fine particles of metal by utilizing above mentioned reducing characteristic.
The inventors of the present invention have took notice of the reducing ability of the silane to the metallic ions which polysilane has, and the stabilizing ability and water solubility caused by the amphipathic polymer reported by the inventors of the present invention, tried to reduce the metallic ions such as gold, palladium or others in water using shell cross-linked micelle having constitutive unit of polysilane in inside as a template, and found that the dispersion of fine particles of metal having controlled particle size (monodispersion) of ten several to several ten nano meter size which are stable in aqueous medium can be obtained. And, the subject of the present invention can be dissolved.